The present invention relates to a circuit arrangement for the power supply of protective devices for vehicle passengers, having a capacitor arrangement which provides the energy necessary for triggering the protective devices.
The passenger protection systems installed in motor vehiclesxe2x80x94these include airbags, seat belts, etc.xe2x80x94are becoming more and more complex over time. Particularly the number of airbags installed in vehicles is becoming ever greater. The restraint devices are to be triggerable in multiple sequential stages in order to offer protection to the passengers even in the event of multiple sequential impacts. In the future, due to the necessary complexity of the protective devices, there will be more than 20 triggering circuits for triggering the individual restraint devices.
In order that all of the restraint devices present in the vehicle can still be activated in the event of a crash even if the power supply in the vehicle is interrupted, an energy store is provided in the vehicle, which supplies the trigger power modules of the individual restraint devices with the necessary energy in case of triggering, as described, for example, in WO 99/29540. An energy store of this type comprises one or more capacitors. As a rule, electrolytic capacitors are currently used, whose overall volume becomes ever larger the more triggering circuits there are to be supplied with energy. In the event of multiple impacts, if a time delay is necessary during the activation of the-individual triggering circuits, the electrolytic capacitor must have a long storage time, which, however, is only possible with a capacitor of an appropriately large design. For example, for a restraint system having 6 triggering circuits and a power consumption of, for example, 100 mA, an electrolytic capacitor having a storage time of approximately 150 ms, with a unit volume of more than 5 cm3, would be necessary. For even more than 6 triggering circuits, the necessary unit volume of an electrolytic capacitor would assume such a size that its installation in an airbag triggering device would be problematic.
If one assumes a restraint system having approximately 20 triggering circuits, the energy store must be capable of providing sufficient electrical energy for at least 5 seconds. For this time, for the number of triggering circuits mentioned, including a microcontroller driving the individual triggering circuits, a total power consumption of 300 mA must be made available. An object of the present invention is therefore to implement the requirements described with an energy store having the smallest possible unit volume.
According to the present invention, a capacitor arrangement used as an energy store comprises multiple double-layer capacitors connected in series and an electrolytic capacitor, which receives its charge from the double-layer capacitors, which have a higher capacitance but a lower nominal voltage than the electrolytic capacitor, and the electrolytic capacitor supplies the triggering voltages necessary for the trigger power modules of the protective devices.
Double-layer capacitors, whose properties are that they have a very high capacitance (more than 7 farads) and a long storage time (greater than 5 seconds), with the unit volume being approximately 3 cm3, are known per se from U.S. Pat. No. 5,621,607. However, double-layer capacitors have a relatively low nominal voltage (approximately 2.3 V), which is too low for triggering a trigger power module of a restraint device. The electrolytic capacitor used in addition to the double-layer capacitors has a higher nominal voltage (approximately 63 V) than the double-layer capacitors and supplies a sufficiently high triggering voltage for the trigger power modules. Since the electrolytic capacitor is exclusively provided for the power supply of the trigger power modules, its capacitance of approximately 1.0 mF, which may be implemented with a relatively low unit volume (6.2 cm3), is reliably sufficient. With the configuration of energy stores according to the present invention, the total unit volume is approximately 18 cm3, which is significantly less (approximately 50 times) than if an energy store for a high number of triggering circuits (e.g., 20) has to be implemented with an electrolytic capacitor.
A voltage transformer is preferably connected to the series circuit of double-layer capacitors, which steps up the voltage which may be picked up at the double-layer capacitors to the charging voltage necessary for the electrolytic capacitor.
A particular advantage is that, in addition to the charging voltage for the electrolytic capacitor, the series circuit of double-layer capacitors also provides the supply voltage for a microcontroller which controls the trigger power modules of the protective device.
An ohmic resistor is expediently connected in parallel to each double-layer capacitor in order to symmetrize the voltages which may be picked up at the double-layer capacitors.